Field
The present invention relates to an image generator and an image generation method for generating an image used in face detection processing of detecting a face from the image.
Related Art
Recently face detection processing of detecting a face from a photographed image obtained by photographing becomes widespread, and is applied to devices such as a digital camera and a smartphone. It is considered that a variation of a face detection processing device increases in future. For example, it is conceivable that the face detection processing is performed with a television set or a monitoring system. For example, when an application of the television set performs face authentication processing or face attribute estimation processing including the face detection processing, a viewer can be identified, or sexuality or age of the viewer can be estimated. As a result, useful information for the viewer (such as recommended program information) can be provided to the viewer. For example, when an application of the monitoring system performs the face authentication processing or face attribute estimation processing including the face detection processing, the detected face is registered for the purpose of crime prevention, a registrant can be authenticated, or the sexuality or age of the registrant can be estimated. As a result, a video analysis can be performed at high speed.
Generally, a light and dark pattern unique to the face (light and dark portions having a positional relationship unique to the face) exists in a facial region of a region of the photographed image. For example, the regions near facial organs such as an eye, a nose, and a mouth include a large number of dark pixels, and other regions include a large number of light pixels. For this reason, in the face detection processing, generally a feature amount is acquired from the image in consideration of the light and dark pattern unique to the face, and the face is detected based on the acquired feature amount. For example, a Haar-like feature amount that is of a difference in luminance between two regions having positional relationship equivalent to the light and dark pattern unique to the face is used as the feature amount.
There is a large amount of information on a light and dark pattern unique to the face in the image in which contrast (difference in luminance between the light and dark portions of the image) remains to a certain extent. Therefore, the face can accurately be detected from the image in which the contrast remains. On the other hand, there is a small amount of information on the light and dark pattern unique to the face in the image having the low contrast. Therefore, the face is hardly detected from the image having the low contrast. Accordingly, the image in which the contrast remains is suitable for the face detection processing.
However, a variation of the photographing environment (for example, lightness around a subject) of the photographed image increases with increasing variation of the face detection processing device. Therefore, the face detection processing is performed using various photographed images from a light photographed image to a dark photographed image, and the face detection processing is performed using the photographed image having the lightness that is unexpected in the conventional face detection processing. For example, sometimes the television set or the monitoring system is used in the dark environment. In the face detection processing performed by the television set, there is a possibility that the photographed image obtained in the dark photographing environment is used.
When the photographing environment changes, a luminance distribution of the photographed image changes, and therefore the contrast changes. As a result, sometimes accuracy of the face detection processing degrades. For example, in the photographed image obtained in the dark photographing environment, a skin region except for a region near a facial organ is also darkened to lower the contrast of the image. Therefore, an information amount associated with the light and dark pattern unique to the face is faded, and the face is hardly detected. Similarly, in the case that the face becomes excessively light by illumination or ambient light, the contrast of the photographed image is lowered, the information amount associated with the light and dark pattern unique to the face is faded, and the face is hardly detected.
Conventionally various pieces of image processing are proposed in order to obtain the desired image. For example, Japanese Translation of PCT Publication No. 2009/063553 and Unexamined Japanese Patent Publication No. 2009-025701 disclose a technology associated with the image processing. However, the secure image (the image in which the contrast remains) suitable for the face detection processing can hardly be obtained even if the conventional technology (such as the technologies disclosed in Japanese Translation of PCT Publication No. 2009/063553 and Unexamined Japanese Patent Publication No. 2009-025701) is used.
For example, in the technology disclosed in Japanese Translation of PCT Publication No. 2009/063553, the image processing is not performed in the case that the whole image is dark. Therefore, even if the technology disclosed in Japanese Translation of PCT Publication No. 2009/063553 is used, the image suitable for the face detection processing can hardly be obtained from the dark photographed image (the photographed image obtained in the dark photographing environment).
In the technology disclosed in Unexamined Japanese Patent Publication No. 2009-025701, only a gamma characteristic (gamma curve) is switched based on whether the number of colors included in the image is greater than or equal to a threshold, but the lightness of the image is not considered. Therefore, the image suitable for the face detection processing can hardly be obtained even if the technology disclosed in Unexamined Japanese Patent Publication No. 2009-025701 is used.